1. Field of the Invention
The present invention relates to a cleaning apparatus used as a processing section in a semiconductor processing system for processing substrates, such as semiconductor wafers and LCD substrates.
2. Description of the Related Art
In a manufacturing process for a semiconductor device such as an LSI, cleaning systems are used to remove contaminants such as particles, organic contaminants and metal impurities on a semiconductor wafer surface. Of these systems, a wet cleaning system is widely used because it can effectively remove particles and enables a batch process.
The wet cleaning system is constructed so as to clean substrates to be processed, such as wafers, with chemicals, e.g., ammonia, hydrofluoric acid, sulfuric acid, and hydrochloric acid, and also with pure water. Cleaning by means of chemicals and pure water is performed in a process vessel in a cleaning section, i.e., a processing section to which various cleaning liquids are supplied.
FIG. 12 shows a conventional process vessel. Cleaning liquid supplying ports 102 and 103 are formed in a bottom portion of a process vessel 101. Disk-like diffusion plates 104 are individually provided above the supplying ports 102 and 103. A rectifying plate 107 having a plurality of holes is arranged horizontally above the diffusion plates 104. A predetermined number of (e.g., 50) wafers w, i.e., substrates to be processed, are supported on a wafer holder 108 located above the rectifying plate 107.
A cleaning liquid supplied through the supplying ports 102 and 103 is diffused in radial directions of the diffusion plates 104, upon striking against the plates. The cleaning liquid is supplied to the process vessel 101 through the holes 106 of the rectifying plate 107. Any overflow of cleaning liquid flows to a receiver 109 provided in the front and rear sides of the cleaning vessel 101. The liquid received in the receiver 109 is circulated by a pump 111 and returned to the supplying ports 102 and 103 through a filter 110.
Bubbles are sometimes generated in a cleaning liquid. The bubbles become greater as they rise upward from the supplying ports 102 and 103, in accordance with the decrease in liquid pressure. If hydrofluoric acid is used as the cleaning liquid, bubbles are liable to be generated in particular and adhere to the surfaces of wafers, resulting in the wafers being cleaned nonuniformly. In addition, since the bubbles are a hotbed of dust particles, the surfaces of the wafers W tend to be contaminated. Since the wafers W are supported with only the edges engaged with grooves formed in the holder 108, they are vibrated by the bubbles. As a result, contaminants are generated from contact portions between the holder grooves and wafer edges.
To prevent contamination, according to the conventional art, bubbles are trapped by the filter 110 and returned to the receiver 109. However, since the cleaning liquid is supplied by the pump 111 with a pressure of about 2 kgf/cm.sup.2, the bubbles become very small due to the pressure, while the liquid is being supplied. Therefore, the filter 110 cannot sufficiently trap the small bubbles in the cleaning liquid.